Working in a Peacemaker Custom 100 which is not quiet at all when the standby switch is not even engaged. The sound is not perfect, but still you can hear the guitar and the master pots do control the signal amplitude. The standby swith has nothing to do with the HV, indeed all the tubes have HV on their plates as soon as the power switch is on. What the standby does is to short the phase inverter outputs wich should produce a cancelation of the signals but that is the problem. The phase inverter is a paraphrase phase splitter circuit and it works great when the standby is pressed but no so good when it should be quiet.
So far what I have seen is that the second triode does not invert in standby so the shorted outputs do not cancel and you can hear it since it is amplified by the power stage. I have isolated the paraphrase from the signal chain just wondering if the masters configuration got something to do but the result was the same, changed the tube with no change in the performance....

I am drawing an schematic since I did not find any info about this amp. I will uploaded ASAP but any hint is welcomed.

A phase-cancelling standby switch works best when the amplitudes of the two outputs are balanced. I suspect in your particular instance, the phase-inverter is not well-balanced. With a constant sinewave input signal at the input of the phase inverter, can you scope the outputs and compare their amplitudes?

I think we have moved past the initial question. The "mute" will only be as good as the gain of a 12AX7. However his mute seems to be not even that good. Last I heard he was looking at the power stage balance.